There is known a linear synchronous motor that is a linear motor using attractive force and repulsive force between magnetic poles of a magnet. As one type of the linear synchronous motor, there is a shaft motor. The shaft motor is disclosed in, for example, a homepage of GMC Hillstone, Co., Ltd.    (http://www.ghc.co.jp/product/shaft.html).
FIG. 1 is a schematic diagram illustrating a configuration of the shaft motor. The shaft motor 101 includes a shaft portion 102 and a coil portion 103. The shaft portion 102 is provided with a plurality of permanent magnets 111 and an outer cylinder 114. The plurality of permanent magnets 111 is arranged along a central axis C such that opposite magnetic directions face to each other in a direction of the central axis C. That is, along the central axis C, N poles are joined to each other, and S poles are joined to each other. For this reason, from joining portions 112, strong magnetic field lines are generated. The outer cylinder 114 is a cylinder that integrally contains the plurality of permanent magnets 111. On the other hand, the coil portion 103 is provided with a coil 118. The coil 118 includes a plurality of coils (e.g.: for U phase, for V phase, and for W phase) which has a common central axis and is respectively applied with AC currents with different phases from one another. The shaft portion 102 passes through an inside of the plurality of coils 118. When the currents flow through the plurality of coils 118 surrounding the shaft portion 102, a magnetic field is generated, whereby thrust force is generated on the basis of the Fleming's left-hand rule. The thrust force allows the coil portion 103 to perform linear motion along the central axis C.
The shaft motor 101 has features, for example, attractive force does not act between the coil 118 and the shaft portion 102, magnetic flux of the magnet 111 can be effectively utilized all around the magnet 111 without wasting to achieve high thrust force, and other features. The shaft motor can be used as a mechanical element substituting a ball screw or as a linear actuator as one of actuators, with use of its functions and features.
As a related technique, a linear synchronous motor is disclosed in Japanese Patent Publication JP2003-70226A. This linear synchronous motor includes a primary mover having a coil and a secondary stator in which a plurality of permanent magnets are disposed along a straight line, and moves the mover along the secondary stator linearly by energization of the coil. In the linear synchronous motor, the plurality of permanent magnets is disposed adjacent to each other, and magnetization directions of the permanent magnets adjacent to each other are shifted by 90 degrees in each moving direction and orthogonal direction of the mover.
Also, Japanese Patent Publication JP2007-6545A discloses a periodical magnetic field generator and a linear motor, a rotary motor and an oscillating motor using the generator. The periodical magnetic field generator includes a field pole of Halbach array structure composed of a main pole permanent magnet magnetized in a direction of generation magnetic field, and a sub-pole permanent magnet magnetized differently from the direction of the magnetic pole of the main pole permanent magnet. In the periodical magnetic field generator, a part of the main pole permanent magnet on a side of the magnetic field generation is replaced by a soft magnetic material.
The inventors have now newly discovered the following fact. It is considered that the above-described shaft motor 101 has the following problems. The first problem is that the magnets 111 are arranged with facing to each other and being in direct contact with each other. Therefore, the magnets 111 facing to each other mutually weaken their magnetic fields. That is, an operating point of the magnet around a contact surface is varied by the influence of the magnetic field generated by the adjacent magnet. Therefore, the magnet is in the state where the magnet cannot exert its performance sufficiently. Also, the second problem is that magnetic flux on the contact surface of the magnet 111 should pass through the inside of the magnets 111 on both sides of the contact surface. Therefore, an intensity of the magnetic flux is limited by saturation magnetic flux densities of the magnets 111. For this reason, it is considered that an intensity of magnetic force of each of the magnets 111 cannot be sufficiently utilized. The third problem is that the magnets 111 are arranged with facing to each other and the adjacent magnets 111 repel each other, thereby being difficult to assemble the motor. In particular, to increase the thrust force, magnetic force of the magnet 111 should be increased. However, if so, it is expected that the assembling becomes increasingly difficult. The fourth problem is that the tubular outer cylinder 114 that contains the magnets 111 is indispensable in order to fix the magnets 111. For this reason, a gap between the coils 118 and the magnets 111 is increased due to the presence of the outer cylinder 114, thereby reducing the thrust force.
In the case of using the above-described shaft motor as a linear actuator, a technique is desired that can sufficiently utilize an intensity of magnetic force of the magnets. A technique is desired that can facilitate the assembling of the shaft motors. A technique is required that can reduce the gap between the coil and the magnets to increase the thrust force.